Flooding

DISCUSSION: On January 8-9, California received a massive amount of rain across much of the state. However, the rain did not bring all good news as the National Weather Service had issued flood warnings as well as flash flood warnings across much of the Los Angeles and Santa Barbara areas. In addition, the National Weather Service had advised that there would be a slight chance of weak waterspouts in the Pacific Ocean near Los Angeles. Northern California was not hit as bad as the storm brought weaker than expected winds to the San Francisco Bay Area. Flooding was minimal in the San Francisco Bay Area as it was mainly minor road flooding with a small closure on California Highway 121 due to a risk of landslides.

It was a different story in Southern California on the morning of January 9th, as a flash flood was observed on the Ventura River near where the Thomas fire was occurring. In addition, mudslides have been reported across Southern California resulting in multiple deaths as well as having portions of U.S. Highway 101 closed. Much of the mudslides have been reported in the Santa Barbara area especially in the higher elevations due to a large amount of rain.​The heavy rain across the state especially in Santa Barbara was due to an atmospheric river event. An atmospheric river event is when moisture was drawn from the warmer portions of the Pacific Ocean near Hawaii and is transported to the coast of California. This atmospheric river event had very high moisture content in the atmosphere up to about 13,000 feet. In addition, the heavy rain in the mountains are also due to the rain shadow effect as moisture condenses as it climbs over mountains and precipitates as the atmosphere cannot hold as much water as the temperature decreases. Forecast models had indicated a very strong upward velocity at both 700 and 850 millibar pressure levels (9500 ft and 5000 ft respectively) in the Santa Barbara mountains during the time of the heaviest rain.

A man stands outside his property near Jordan Creek on October 27, 2017

During the week of October 23rd, hints of a strong atmospheric river (a narrow band of concentrated moisture in the atmosphere) were present in the extended forecast for southeast Alaska, as moisture from super typhoon Lan was expected to push through. Typhoon Lan formed in the northwest Pacific on October 14th, affecting the Philippines, Japan, and South Korea and causing several deaths along its track. By October 23rd, Lan was showing signs of dissipation. The remnant moisture fueled the atmospheric river, bringing heavy flooding to Alaska.

On Wednesday, October 25th, an areal flood watch had been issued for the northern half of southeast Alaska due to projected small stream rises. As rain began on Thursday, a flood advisory was in effect, where rivers reached bankfull and high freezing levels (>6000 ft) increased the threat for runoff and higher elevation snowmelt. Early Friday morning, heavy rains and strong winds near 40 mph continued to impact the Skagway and downtown Juneau areas. The 48-hour rainfall totals for Thursday and Friday ranged from 3.78” at Yakutat Airport, 4” at the Glacier Bay Fuel Dock in Gustavus and at the National Weather Service office in Juneau, 4.5” for downtown Juneau, 6” at Sawmill Creek in Sitka, and 8” in Pelican.

By 10 a.m. on Friday, October 27th, a flood warning was issued for Juneau Creek near Juneau. Jordan Creek usually reaches action stage at 9.2 ft, flood stage at 9.7 ft, and moderate flood stage at 10.5 ft. It reached a record crest of 11.0 ft by 2:30 p.m. local time on Friday. The last historic crest of Jordan Creek occurred on September 9, 2016 at 10.18 ft. As a result, several residential homes, office buildings, and roads near Jordan Creek (and also Salmon Creek in Gustavus) were inundated by a foot of water. A mudslide occurred near the Mt. Roberts Tramway system in Juneau, closing roads and delaying traffic.

For more about flooding and other applied meteorology topics, please click here!

DISCUSSION: In light of the 2017 Atlantic hurricane season being a very high-impact year with respect to widespread flooding occurring from a combination of both storm surge and heavy rainfall, there are messages to be learned from such events. First off, in light of major flooding events which occurred in association with tropical cyclones such as Hurricane Harvey and Hurricane Irma, many people are always wondering whether they would ever find themselves within a major flood evacuation zone for a future winter and/or tropical storm. Having said that, it is useful and important to know that there are in fact ways to find out such information. By going to websites such as the following link, you can take a virtual tour of places around the world to see the relative sensitivity of the location to annual flooding potential. Moreover, being cognizant of potential flooding threats (regardless of the given season) is particularly important since this allows someone to be able to better prepare and protect themselves from future flooding events. Whether it is through taking precautionary measures to protect life and property while holding one's ground or taking as much as possible and getting out of town, there are always way to both prepare for and/or respond to flooding event situations.

To learn more about other flooding-related stories from around the world, be sure to click here!

DISCUSSION: Over the past 24 to 36 hours, a small (but potent) piece of mid-level energy propagated across portions of west-central Europe and moved across areas which included (but were certainly not limited to) far western Austria, eastern Switzerland, and far southern sections of Germany. Across many cities/towns located in these parts of those respective countries there was a clear focal point for strong convection which fired up during the day on Saturday. Having said that, there was also a consistent explanation for the particularly focused nature of this regional European flooding (and in some cases the flash flooding) event which unfolded during the day on Saturday. This explanation can be best characterized by the areas of elevated terrain helping to facilitate the greater likelihood of heavier rainfall on a more localized scale. This is due to the fact that the more elevated terrain across these parts of those respective countries forces air parcels to more abruptly move up and over such topographic barriers (e.g., hills, plateaus, mountain ridges, etc.).

During such orographic-enhancement of precipitating-clouds, there is even more liquid water "squeezed" out of developing convection via increased instability due to the air parcels rising and being forced to expand more and more with increasing height. Thus, this favorable convective combination allowed for notably heavier rainfall to unfold in cities such as Tux, Austria as captured in the raw footage above. Events such as the major flash flooding event captured above just yesterday afternoon (local time in western Austria), reinforce the importance of always remaining weather-ready at all times regardless of your location, plans, or individual circumstances prior to or during the occurrence of the flooding.

To learn more about other high-impact flooding events from around the world, be sure to click here!

On Saturday, July 31, 1976, several hundred tourists and locals were enjoying a beautiful summer afternoon in the resort town of Estes Park in northern Colorado. A stationary front (when warm and cold air masses meet and neither is strong enough to move the other) had settled in an east-west fashion across Colorado, Kansas, and Missouri. Warm, moist air was forced up the Front Range and the unstable air continued rising as the water vapor condensed, becoming conditionally unstable (where the air is unstable if saturated but stable if saturated).

Light rain began falling over the Colorado Rockies during the late afternoon. By 7:35 p.m., the National Weather Service had issued a severe thunderstorm warning. Around 9:00 p.m., the National Weather Service issued an urgent warning to expect flooding in the Big Thompson Canyon, but the warning came too late for most. At this point, 12 inches of rain had fallen within the last four hours and sent a 20-foot wall of water down the narrow canyon, causing flood waters to speed up as they squeezed through. (According to the Bernoulli principle fluids speed up as they are forced through a tight space due to an increase in pressure). Weaker winds aloft made the thunderstorm nearly stationary near the upper section of the canyon.

Rex Sheets of Loveland, Colorado was driving with his friend up towards Lake Estes to go fishing on July 31st. A state patrolman began driving up the canyon during the early evening hours to warn others of the storm. The frame of his car was later found with the steering wheel attached and his keys still in the ignition. The patrolman’s body was never recovered.

The amount of rain that fell in that short amount of time made it so that the rain wasn’t able to soak into the canyon sidewalls. Boulders the size of cars were carried down the river by the floodwaters and houses were transported on top of bridges. The flood caused $35 million in damage, destroyed 418 homes, and claimed the lives of 143 people.

DISCUSSION: Over the past 24 hours, a storm similar to a Nor’easter dropped an immense amount of rain in the Mid-Atlantic region. Highly unusual for this time of year, this coastal storm (with a track that is typical of a winter Nor’easter) carried lots of tropical moisture which allowed it to drop nearly 6-7 inches of rain near Washington, DC. Rain gauges in parts of southwestern Delaware measured more than 10 inches of precipitation! While the rain is still continuing, a map detailing 24-hour precipitation totals for the region showcases the effects of this coastal storm (see above, courtesy of NWS WPC).

Early this morning the NWS Weather Prediction Center released a Mesoscale Precipitation Discussion (MPD #:0592) warning the Delmarva area of the threat of flash flooding from this coastal storm. The graphic they released for the MPD (below, courtesy of NWS WPC) shows the conditions that were set up for the storm. A low pressure center with counterclockwise winds around it was riding along the coast. This allowed for warm, moist air from the south to mix into the stationary front, providing it a source to draw more energy from, and cooler air from the northeast to drive strong winds into the area. The storm had a history of heavy rains, and with convection that continued to pop up along and in front of the stationary front, the storm had another source to draw energy and moisture from that permitted it to slow down and downpour.

​In addition, the coastal storm had the help of the unusually sharp contrast of sea surface temperatures in the Atlantic for this time of year, (see below, courtesy of NWS Eastern Region HQ) which is typically a driving force for Nor’easters. This contrast also shows the Gulf Stream pushing further north than we usually see it. With the combination of all of these conditions, rain poured at large rates over a long duration, allowing for incredible amounts of precipitation for the area.

To learn more about other flooding events from around the world, be sure to click here!

Torrential rain has hit parts of Kansas and Missouri prompting water rescues. Radar indicates 3 to 7 inches of rain has fallen during this event with flash flooding imminent. Indian Creek in Kansas City has risen over 10 feet in 4 hours. Tomahawk Creek is also a concern for the people living along the banks. Water rescues are ongoing at the time of this article and will continue as water levels are expected to continue to rise even though the rain has ended. Buildings have been closed due to flooding as well as road closures including major highways in the area. Officials are asking people to not drive through flooded roadways for safety. The complete Flash Flood Warning from the National Service in Pleasant Hill/Kansas City, Missouri is as follows:

“* Flash Flood Warning for... Wyandotte County in northeastern Kansas... Southeastern Leavenworth County in northeastern Kansas... Johnson County in east central Kansas... Clay County in west central Missouri... Southern Ray County in west central Missouri... Lafayette County in west central Missouri... Eastern Platte County in west central Missouri... Jackson County in west central Missouri... Northern Johnson County in west central Missouri...

* Extensive flooding has occurred near Indian Creek with water over the road at 103rd and Wornell Rd and multiple water rescues. Indian Creek has crested and is starting to fall. The Blue River has also experienced Minor to Moderate flooding and is starting to crest currently. Tomahawk Creek reached record flooding overnight but has crested and is starting to fall.

DISCUSSION: Widespread flash flood watches were issued Sunday across portions of the Mid-Atlantic from Maryland to New York. There was plenty of available moisture in the atmosphere, as precipitable water values (or PWATs, as they are often referred to) reached upwards of two inches across the Mid-Atlantic coastline, seen below. Remember that precipitable water indicates the amount of rain that would fall if all of the moisture were to be squeezed out of a column of air.

At the 300mb pressure level, a potent trough was driving through the Great Lakes region. Take a look at the purple contours below, which indicate divergence. If air diverges at upper levels, then that means air from lower in the atmosphere must rise to take its place. This is important to the flash flooding setup, as moisture-laden air that is forced to rise can create thunderstorms with torrential rains.

A bit closer to the surface, at 500mb, we can still see the trough anchored in the Ohio Valley, but we can also see vorticity as shown by the green and yellow shadings. Vorticity is a measure of fluid rotation, and an area of vorticity moving towards a particular location is another sign of rising air. For this situation, note the vorticity maximum near Atlantic City (marked as an "X"). Over the previous four hours, that "X" had migrated northeastward from the Washington DC area, meaning that air would be forced upwards above Delaware and southern New Jersey.

Combine the high PWATs, upper-level trough, and mid-level vorticity movement, and you've got the perfect recipe for flash flooding. Check out the loop below, courtesy of COD Meteorology, as storms quickly form in Delaware and southern New Jersey - the exact areas where upper-level divergence and rising air were at a maximum. It's a satisfying feeling to see the principles you learn about play out in real time.

Here's a 90-minute radar loop showing the rapid growth of localized storms last night as a trough approached from the south. pic.twitter.com/JDErOd5s4n

While these were the conditions between roughly 11PM and midnight Eastern time, dangerous flooding had already occurred earlier on Sunday. A slow-moving storm in northern Delaware and extreme southeastern PA dumped over four inches of rain in spots, with two inches falling in a single hour in the town of Chadds Ford, PA. Not to be outdone, the airport in Harrisburg, PA measured an amazing 4.27 inches in one hour, in which 1.83 inches fell in just 21 minutes! These totals were extremely localized, however, as rainfall amounts greatly diminished only five miles away from each of these locations. Even high-resolution models can't predict the weather on that small of a scale, so it's very important to keep an eye on the radar whenever a flash flood situation is possible.

To learn more about other flooding events from around the world, be sure to click here!

DISCUSSION: After the recent major flooding event which severely impacted several states to the west of the Mississippi River, there were many instances of widespread damage across the region. As shown in the footage above, several counties across the state of Arkansas were quite severely impacted by persistently heavy rainfall over the course of several consecutive days. It got to such a bad extent that the National Guard needed to be called in to help relieve some of the larger pressures placed on this part of the United States in the wake of such an extensive flooding event. It is also important to note that in the wake of such a major and widespread flooding event, one should always aim to avoid any and all flooded roadways due to its inherent ability to create debris flows which can include various infrastructural debris as well as various automobiles.

To learn more about other flooding events from around the world, be sure to click here!